The proposed investigations are directed towards in depth study in animal and human intestine of the oppositely directed absorptive and secretory processes which together determine net intestinal transport of water and electrolytes. These studies will evaluate: 1) The place of the intestinal mucosal enzyme Na-K-ATPase in basal absorptive and secretory processes in vivo, and in active Na and K transport in vitro; 2) New aspects of the pathophysiology of diarrhea. Specifically, the mechanism of serotonin-induced water and electrolyte secretion in the rabbit ileum; particularly whether active electrolyte secretion, altered intestinal permeability, or altered neural control are responsible. 3) Whether a pharmacologic approach to the treatment of diarrheal diseases can be developed by using agents which increase intestinal absorption and/or decrease intestinal secretion. The mechanism by which the glucocorticosteroid methylprednisolone increases intestinal absorption and Na-K-ATPase activity, and the potential usefulness of this agent in the management of clinical diarrheal diseases, will be studied. Also whether the B-adrenergic receptor blocker propranolol can alter active intestinal secretion. The methods that will be used to study water and electrolyte transport and the enzymatic mediators involved will include: a) in vivo the single pass perfusion technique; b) in vitro the Ussing chamber and voltage clamp technique; and c) determination of activities of the intestinal mucosal enzymes: Na-K-ATPase, Mg-ATPase, adenylate cyclase and cyclic nucleotide phosphodiesterase.